Lactones, such as .epsilon.-caprolactone, form polymers which have the characteristic of being compatible with polymers such as styrene-acrylonitrile, polymethyl methacrylate, vinyl chloride polymers and the like. In order to increase the range of applications of lactone polymers, attempts have been made to prepare copolymers in which the lactone polymer is bonded chemically with another conventional polymer. However, no process is known in the art which would permit the preparation of such copolymers at satisfactory yields and on a large scale.